Dropping on the 16th of July 2005, Harry Potter and the Half-Blood Prince yet again saw even fewer signed copies surface, meaning anyone that owned a pristine true first print copy of Harry Potter and the Half-Blood Prince could be looking at hundreds if not thousands of pounds if they sold it.

Spoilers ahead. The whole reason Harry hides the book in the room of requirement is because Snape found out from his spell on Malfoy which is ultimately how he finds the diadem in HP7. It also teaches him about he Bezor which is how he saves Ron and it's the reason he gets the memory from Slughorn via Felix Felicis.

Look at the publishing information page. It will tell you if it's first edition, as well as what print run it is. If you're not sure how to interpret the information, I've seen some guides online, or you can post a picture here.

Harry Potter and the Order of the Phoenix is the longest book in the series, at 766 pages in the UK version and 870 pages in the US version. It was published worldwide in English on 21 June 2003.